Water: Evaporation Rate

Let's talk a little about evaporation rate. I'm going to define evaporation rate (or 'boil-off') as the volume of water that evaporates during a vigorous boil in a 1-hour period. This can be written as an absolute volume ('3 quarts') or as a percentage ('15%').

Why does this matter? If you are doing partial boils, it doesn't. You are going to top off your partial boil to the desired final volume anyway, so it really only affects how much you need to add. As long as you prepare more than you think you'll need (by adding gypsum, campden, pre-boiling, whatever) you'll be fine.

It matters a lot when you're doing full boils and all grain, though. If you want consistency and peace of mind, you need to figure out exactly how much water you'll be using, start to finish. BeerSmith has great tools that let you do this, but you need to give it accurate numbers first. Once you've configured BeerSmith, it will calculate exactly how much water you need to prepare, how much to mash and sparge with, how much will go into your boil kettle, and how much will come out.

The biggest problem with BeerSmith is that it treats evaporation rate as a percentage of total volume. Evaporation doesn't work that way. Evaporation depends on the heat (and thus, vigor) of the boil, atmospheric pressure and temperature, and the surface area of the water exposed to air. It does NOT depend on how much water you started with. If you boil 1 gallon of water in an hour when you start with 4 gallons, you will still boil off 1 gallon of water if you only have 1 gallon the pot to begin with.

I've made this mistake before, twice. First, I used a reasonable default of 15%, and ended up boiling much more than that. It turns out, the 15% value was for full boils of 5 gallon batches (7-8 gallon boil volume). Since my boils were actually half that, the 'percentage' was also half. This is a big clue that using a percentage makes no sense.

The second time, I calculated the evaporation rate of 3 gallons of water boiling for an hour, using my 6 gallon pot, to be about 0.9 gallons. This is 30% of the original 3 gallons. I plugged this into BeerSmith and it had me boil over 4 gallons. Unfortunately, 30% of 4-odd gallons is more than 30% of 3 gallons. I ended up still only boiling off 0.9-1 gallon, and had about a quart of extra water that never boiled off.

The best solution would be if BeerSmith stopped using percents for evaporation rate and allowed you to put in quarts/hour or gallons/hour. Until they fix that, here's what you do:

• Fill a pot with approximately what your typical boil volume will be. It doesn't have to be exact. For example, 4 gallons for a half batch full boil.
• Boil for an hour. Use the same heat level as you would during a real wort boil.
• Immediately remove from heat and place the cover on the pot. Let it cool.
• Carefully measure the remaining volume. Subtract the remaining volume from your starting volume. Call this your boil-off volume. For example, let's say you started with 4 gallons and ended up with 3 at the end. The boil off would be 1 gallon (4 quarts).
• Open up the equipment configuration screen in BeerSmith. Find the two boxes labeled Evaporation Rate and Boil Off. You can change Evaporation Rate, but not Boil Off. 
• Make sure you have configured your other values properly - top up water (0 for me), Final Volume (11 quarts for me) and Lost to Trub and Chiller (0 for me).
• Type 30 into the Evaporation Rate box. Notice that the Boil Off value changes automatically, and is probably way too high. For me, it reads 4.91 quarts.
• Keep adjusting the value in the Evaporation Rate box until it reaches exactly how much boil off you measured in your experiment. I tweaked the number down until the Evaporation Rate box read 25.86% and the Boil Off box read 4 quarts. Do this *every* time you change your final volume.
• The Boil Volume should now read the amount of water you need at the beginning of a one-hour boil, if you have checked Calculate Boil Volume Automatically. For me, it now reads 15.46 quarts.
• BeerSmith will now accurately tell you how much to sparge with and how much boil volume to use! Double check this volume on brew day - you may lose more or less water due to your own process, the time of year or the phases of the moon. Tweak this value up and down until you get consistent results every time. 

If you're curious, my Winco 20 quart stock pot has an evaporation rate of about 2.75 quarts in an hour or 4.14 quarts in 90 minutes.